CLIMATIC FACTORS : THE AIR 



95 



wise be lost to the air. Several inches of the total rainfall may, 

 therefore, be of no significance whatever except to raise the hu- 

 midity temporarily and reduce transpiration for a short time. If 

 rain falls heavily for short periods, say two or three inches in the 

 same number of hours, much of it will be lost by runoff, the 

 amount varying with steepness of slope, nature of the soil, and 



Grassland Deciduous Forest Deciduous Forest Coniferous Forest 



Cheyenne. Wyo. Indianapolis. Ino. Richmond.Va Ottawa, Canada 



Jan. 



JULY 



FlG. 42. Annual precipitation patterns (based on averages) for several 

 stations, which illustrate the relative amounts and distribution of precipitation 

 throughout the year for areas supporting grassland, deciduous forest, and 

 coniferous forest.— A dapted jrom Transeau. 2 



256 



amount and kind of cover. Again, the seasonal distribution of 

 rainfall may be of much more importance than the total amount. 

 If rainfall is uniformly distributed throughout the growing sea- 

 son, moisture conditions may be far more favorable with twenty- 

 five to thirty inches than they would be with forty to forty-five 

 inches if the growing season is interrupted by one or more pro- 

 tracted dry spells. If precipitation is regularly seasonal, the type 

 of vegetation may be definitely limited. For instance, grasslands 

 characterize those areas where rainfall is rather light and con- 

 centrated in the spring and early summer. Winter rains with dry 

 summers, characteristic of several coastal regions, support shrub- 

 by vegetation. 



Measurement of Precipitation.— A standard rain gauge is a cyl- 

 inder 8 inches in diameter and 20 inches high, which has a funnel 

 built into the upper end that permits the water it catches to run 

 into an inner cylinder with exactly one-tenth the cross-sectional 

 area. The ratio of the outer to the inner cylinder being 10:1, the 

 measurement of water collected in the tube must be divided bv 

 ten if taken directly, or it can be measured with a standard 



